The present invention relates to a process for the manufacturing of products made of thermoplastic material, which products have a high creep strain resistance without the need for reinforcing additives.
Products made of thermoplastic materials can be manufactured by a number of different manufacturing procedures. The most commonly used methods are however, injection moulding, vacuum forming, blow moulding and press moulding.
In some fields of application a high load carrying capability is required. Carrying designs made of materials such as steel and concrete will be able to handle a load almost irrespective of factors like time and temperature. This is not the case with thermoplastic materials where relatively small loads can cause a remaining deformation when charged for a long period of time. This phenomenon is called creep strain, creep deformation or creepage. This creep strain is accelerated if the temperature is raised. A design made of thermoplastic material will however be able to withstand loads that are tens of times higher without remaining deformation when charged for shorter periods of time. The relation between the amount of creep strain, time and temperature is depending on type and quality of thermoplastic material.
Carrying thermoplastic designs most often have to be designed to withstand the highest temperature together with the longest period of time and the highest load that it could be exposed to during its useful life. The creep strain can however be decreased by adding filling or reinforcing additives to the thermoplastic material. Among filling additives that are commonly used can be mentioned minerals such as lime, glass beads and mica while reinforcing additives that are commonly used are fibres such as glass fibres, steel fibres or carbon fibres. It is also known to reinforce a thermoplastic product by integrating a metal design with the product. This can for example be constituted by a steel rod applied in a profile in the thermoplastic product. These additives and additions will however decrease some of the good qualities naturally occurring in the thermoplastic material. Among those qualities can be mentioned good impact strength, low weight and being a good electric, acoustic and heat insulator. It will also principally be impossible to recycle the material from a product containing additives. The ability to recycle thermoplastic materials is principally compulsory nowadays.
It has, quite surprisingly, according to the present invention, been made possible to manufacture creep stain resistant products of thermoplastic material, without the need of reinforcing additives. Accordingly, the invention relates to a process for the manufacturing of thermoplastic products with a high resistance against creep strain although the products are free from reinforcing additives. The process includes at least vacuum forming and/or blow moulding of a thermoplastic material such as polyethylene, polypropylene or polybutene. The invention is characterised in that a number of product parts are manufactured from a number of preferably tube or sheet shaped work pieces. The work pieces are heated so that the thermoplastic material softens, whereupon they are given the desired shape by means of a mould and the influence of vacuum and/or pressure. The product parts produced are allowed to cool and post-shrink. The product parts are hereafter assembled to a unit.
As is commonly known in the art of moulding plastics, post-shrinking is distinct from simply cooling. Specifically, a moulded part is considered to be post-shrunk when there is no longer a dimensional change of the moulded part conditioning for a specific period at a specific temperature.
According to one embodiment of the invention, at least one of the product parts is manufactured through injection moulding by means of a mould comprising one or more mould cavities. A molten thermoplastic material is injected into a mould cavity of the mould. The thermoplastic material is allowed to solidify. The mould can then be opened and product part be removed from the mould. The product part is then allowed to cool completely and post-shrink. The product parts is alternatively manufactured through injection moulding by means of a mould comprising one or more mould cavities. The mould includes means for injecting a pressurised gas. Molten thermoplastic material is injected into a mould cavity of the mould, whereupon the pressurised gas is injected into the molten thermoplastic material in the mould cavity. The thermoplastic material is allowed to solidify whereupon the gas is evacuated. The mould is then opened and the product part produced is removed from the mould. The product part is allowed to cool completely and post-shrink. It will hereby be possible to manufacture special features such as for example label pockets which are integrated with the article. Such special features are normally not possible to manufacture through vacuum forming or blow moulding.
According to one embodiment of the invention, one or more of the product parts are manufactured in a mould comprising a first and a second mould half. The mould halves include one shape-giving cavity each. The two shape-giving cavities together form a negative depiction of the outer shape of a product part. The mould halves are placed so that a space is formed between the two mould halves and so that the mould cavities are directed towards each other. Two pre-heated sheet shaped work pieces are applied between the two mould halves. The work pieces are individually forced towards the shape-giving surface of the mould cavities by means of vacuum and/or pressure. The mould halves are pressed together while the thermoplastic material is still hot so that the material in the work pieces confounds and a hollow unit is formed. It has shown to be a great advantage to connect the two surfaces on either side of the intermediate hollow space with each other by means of locally placed ridges or tower-like parts on product part containing large straight surfaces. This will increase the mechanical stability of the product as well as the ability to withstand bending when a load is applied.
The product parts preferably form parts such as a deck, a foot or a skid of a pallet, or a deck, a foot, a side wall or a skid of a pallet container or the like. According to one embodiment of the invention, the injection moulded product parts forms parts such as a foot or a skid to pallet, a foot, a side wall or a skid to a pallet container. These product parts are joined by welding such as butt welding, friction welding or filler welding. The surface of two product parts that are to be joined are heated until they melt when utilising the butt welding procedure. The heating is preferably achieved by bringing the surfaces to be joined in contact with a heated metal plate. The heated product parts are then pressed towards each other while the melted surfaces are allowed to cool.
When utilising friction welding, the surfaces that are to be joined are rubbed until they melt due to the friction heat. The most commonly used variants of this method is ultra-sonic welding, low frequency welding and rotation welding.
Filler welding used on thermoplastic materials is similar to gas welding with filler bar used on metal. The surface of the joint and a filler bar made of the same thermoplastic material as in the product parts are heated with a hot air blower. The filler bar is used for filling the joint in a manner similar to that used for metal welding. The latter method can also be used in combination with the above mentioned methods.
The thermoplastic material used is preferably constituted by a polymer such as polyethylene, polypropylene or polybutene with an average molecular mass in the range 200,000-2,000,000 preferably above 300,000. In certain cases, such as for example when manufacturing products with thin walls, polymers with an average molecular mass in the range 1,000,000-2,000,000 could show advantageous, while products with heavy walls most often are manufactured of a polymer with an average molecular mass in the range 300,000-1,000,000.
According to one embodiment of the invention the tube or sheet shaped work pieces are constituted by a thermoplastic laminate with two or more layers. The layers are constituted by a combination of two or more of the materials selected from the group, virgin solid thermoplastic material, re-cycled solid thermoplastic material, virgin expanded thermoplastic material and re-cycled expanded thermoplastic material.